1. Field of the Invention
The present invention relates to oxide thin films produced by an ALD method. In particular, the present invention relates to yttrium-stabilised zirconium oxide (YSZ) thin films.
2. Description of Related Art
The continuous decrease in the size of microelectronic components leads to the situation in which SiO2 used today as the gate oxide in metal oxide semiconductor field effect transitions (MOSEFT) must be replaced with a higher permittivity oxide. This is due to the fact that in order to achieve the required capacitances, the SiO2 layer should be made so thin that the tunneling current would increase to a level affecting the functioning of the component. This problem can be solved by using a dielectric material having a higher dielectric constant than SiO2. For example, the capacitance of dynamic random access memory (DRAM) capacitors must remain nearly constant while their size decreases rapidly, and thus it is necessary to replace the previously used SiO2 and Si3N4 with materials which have higher permittivities than these and give higher capacitance density.
There is a number of materials exhibiting sufficiently high dielectric constant, but in addition to high permittivities, these dielectric thin films are required to have, among other things, low leakage current densities and high dielectric breakdown fields. The achievement of both of these properties presupposes a dense and flawless film structure. It is also important that the materials are stable in contact with silicon and can be exposed to the high post-treatment temperatures essentially without changes. Especially in the gate oxide application it is important that in the interface between silicon and the metal oxide having high dielectric constant there are very few electrically active states. In the memory application it is important that the structure of the dielectric of the capacitor is stable, since the temperatures used for activation of implanted ions are high.
Zirconium oxide, ZrO2 is an insulating material having a high melting point and good chemical stability. ZrO2 can be further stabilised by adding other oxides, the aim of adding other oxides is to eliminate the phase changes of ZrO2. Normally, the monoclinic crystal form is stable up to 1100xc2x0 C. and tetragonal up to 2285xc2x0 C., above which the cubic form is stable. The stabilisation is typically carried out by adding yttrium oxide (Y2O3), but also MgO, CaO, CeO2, In2O3, Gd2O3, and Al2O3 have been used. Previously, YSZ thin film layers have been produced, for example, by metal-organic chemical vapour deposition (MOCVD) (Garcia, G. et al., Preparation of YSZ layers by MOCVD: Influence of experimental parameters on the morphology of the film, J. Crystal Growth 156 (1995), 426) and e-beam evaporation techniques (cf. Matthee, Th. et al., Orientation relationships of epitaxial oxide buffer layers on silicon (100) for high-temperature superconducting YBa2Cu3O7xe2x88x92x films, Appl. Phys. Lett. 61 (1992), 1240).
Atomic layer deposition (ALD) can be used for producing binary oxide thin films. ALD, which originally was known as atomic layer epitaxy (ALE) is a variant of traditional CVD. The method name was recently changed from ALE into ALD to avoid possible confusion when discussing about polycrystalline and amorphous thin films. Equipment for ALD is supplied under the name ALCVD(trademark) by ASM Microchemistry Oy, Espoo, Finland. The ALD method is based on sequential self-saturating surface reactions. The method is described in detail in U.S. Pat. Nos. 4,058,430 and 5,711,811. The growth benefits from the usage of inert carrier and purging gases which makes the system faster.
When ALD type process is used for producing more complicated compounds, all components may not have, at the same reaction temperature range, an ALD process window, in which the growth is controlled. Mxc3x6lsxc3xa4 et al. have discovered that an ALD type growth can be obtained when growing binary compounds even if a real ALD window has not been found, but the growth rate of the thin film depends on the temperature (Mxc3x6lsxc3xa4, H. et al., Adv. Mat. Opt. El. 4 (1994), 389). The use of such a source material and reaction temperature for the production of solid solutions and doped thin films may be found difficult when a precise concentration control is desired. Also the scaling of the process becomes more difficult, if small temperature changes have an effect on the growth process. Mxc3x6lsxc3xa4 et al. (Mxc3x6lsxc3xa4, H. et al., Adv. Mat. Opt. El. 4 (1994), 389) disclosed a process for growing Y2O3 by ALE-method. They used Y(thd)3 (thd=2,2,6,6-tetramethyl-3,5-heptanedione) as the yttrium source material and ozone-oxygen mixture as the oxygen source material in a temperature range of 400-500xc2x0 C. As already discussed, no ALE window could be found since the growth rate increased steadily from 0.3 xc3x85/cycle to 1.8 xc3x85/cycle with increasing temperature.
Ritala et al. (Ritala, M. and Leskelxc3xa4, M., Appl. Surf Sci. 75 (1994), 333) have disclosed a process for growing ZrO2 by an ALD type process. ZrCl4 was used as the zirconium source material and water was used as the oxygen source material. The temperature in the process was 500xc2x0 C. and the growth rate was 0.53 xc3x85/cycle.
It is an object of the present invention to eliminate the problems of prior art and to provide a novel process for producing yttrium-stabilised zirconium oxide (YSZ) thin films.
This and other objects together with the advantages thereof are achieved by the present invention as hereinafter described and claimed.
The present invention is based on the finding that yttrium oxide and zirconium oxide can be grown by an ALD type method so that the film growth is in accordance with the principles of ALD so as to form an yttrium-stabilised zirconium oxide thin film. More specifically, the process for producing YSZ thin films is characterised by what is stated in the characterising part of claim 1.
A number of considerable advantages are achieved by means of the present invention.
The growth rate of the yttrium-stabilised zirconium oxide thin film is high, e.g., the growth rate of ALD thin film was approximately 25% higher than would be expected based on the growth rates of ZrO2 and Y2O3.
The temperatures used in the present invention are low compared with the processes of prior art, which reduces the cost of the production process.
A film grown with the present process exhibits good thin film properties. Thus, the oxide films obtained have an excellent conformality even on uneven surfaces. The method also provides an excellent and automatic self-control for the film growth.
The ALD grown yttrium-stabilised zirconium oxide thin films can be used, for example, as insulators in electronics and optics. For example, in field emission displays (FED) it is preferred that insulating oxides which have a smooth surface, are used. It is also possible to use the YSZ thin films as solid electrolytes in gas sensors and fuel cells. Particularly suitably the YSZ thin films are used as gate oxides in microelectronic devices, and as capacitor in dynamic random access memory (DRAM).
Next, the invention is described in detail with the aid of the following detailed description and by reference to the attached drawings.